U.S. Pat. No. 4,833,909 discloses an apparatus for detecting a gaseous componet in air. The apparatus includes a measuring chamber through which a throughflow can take place from a gas inlet to a gas outlet. The gas sample to be investigated is guided through a sample intake line to the gas inlet of the measuring chamber. A pressure is built up in the sample intake line at the inflow end by means of a throttle and this pressure is detected by a pressure sensor. The pressure measurement signal supplied by the pressure sensor is an index for the velocity of the gas flow within the gas sample intake line. A value proportional to the gas volume can be determined by a time integration of the pressure measurement signal.
Arrangements are known wherein a measurement value is needed for calibration purposes and this measurement value is proportional to the ambient pressure. Test gases from pressure cylinders are used for the calibration and these test gases are fed into the measuring apparatus to be checked. Generally, the test gas concentration is indicated in ppm or volume percent on the pressure cylinders and the test gas concentration is thereby independent of pressure. The concentration indication on the pressure cylinders and the value supplied by the measurement apparatus are only coincident for the ambient pressure at which the measuring apparatus was adjusted with this test gas. With changes of the ambient pressure, a difference arises which must be corrected. An arrangement for correcting the ambient pressure influence is disclosed in U.S. Pat. No. 5,400,637.